1. The Field of Invention
The present invention relates to improvements in the systems and methods for non-invasively measuring one or more biologic constituent concentration values. More particularly, the present invention relates to non-invasive spectrophotometric systems and methods for quantitatively and continuously monitoring the hematocrit and other blood parameters.
2. The Prior Art
Modern medical practice utilizes a number of procedures and indicators to assess a patient's condition. One of these indicators is the patient's hematocrit. Hematocrit (often abbreviated as HCT) is the volume expressed as a percentage of the patient's blood which is occupied by red corpuscles, commonly referred to as red blood cells. The present invention is presented in the context of hematocrit. However, it is to be understood that the teachings of the present invention apply to any desired biologic constituent parameter.
Medical professionals routinely desire to know the hematocrit of a patient. In order to determine hematocrit using any of the techniques available to date, it is necessary to draw a sample of blood by puncturing a vein or invading a capillary. Then, using widely accepted techniques, the sample of blood is subjected to either high-speed centrifuge, cell counting, ultrasonic, conductometric or photometric methods of evaluating the sample of blood in a fixed container. Prior U.S. Pat. No. 5,372,136 indicates a system and methodology for determining the hematocrit non-invasively, without puncturing or invading the body, spectrophotometrically and continuously in a subject. The present invention relates to improvements upon the above cited system.
Beyond the above referenced patent, others have suggested various means of noninvasive measurement of hematocrit. Specifically, Mendelson, U.S. Pat. No. 5,277,181; Seeker, U.S. Pat. No. 5,188,108; Gonatas, U.S. Pat. No. 5,528,365; Ishikawa, U.S. Pat. No. 5,522,388; Shiga, U.S. Pat. No. 4,927,264; Tsuchiya, U.S. Pat. Nos. 5,441,054, 5,529,065, 5,517,987 and 5,477,051; and Chance, U.S. Pat. Nos. 5,353,799, 5,402,778, and 5,673,701 have attempted to define means of directly measuring desired biologic constituents such as hematocrit. Even though the various patents indicate the need to utilize multiple wavelengths measured at different detection sites and/or the need to perform differential or ratiometric operations on the detected optical signal, all fail to isolate and resolve the individual and specific scattering and absorption coefficients of the desired constituent. At best they address only bulk attenuation coefficients and/or bulk diffusion constants of the scattering media while attempting to resolve such constraints as tissue nonhomogeneity. As an example, tissue may be considered to contain a bulk absorptive coefficient due to blood, collagen, water, fibers, bone, fingernail, etc. Hence, in order to determine the absorptive coefficient of the blood itself, the bulk value of the tissue per se must be prorated by the amounts of the above constituents. Secondly, the actual absorptive coefficient of the blood must then be decoupled or isolated from its proration factor as well.